Fluidised bed apparatus

ABSTRACT

A fluidized bed forming apparatus is provided which includes a filter extending partly across a base to define a plenum chamber beneath the filter. At least two end base portions are not above the plenum chamber and no fluidization occurs above those portions. Means are provided for supplying powder on to the base to form a fluidized bed which is retained by the unfluidized powder over those end base portions and else where either by solid walls or further walls of unfluidized powder.

United States Patent Jones 1 May 20, 1975 [54] FLUIDISED BED APPARATUS 3,593,678 7/1971 Miller 118/630 3,809,011 5/1974 Fabre et a1 ll8/D1G. 5 [761 Inventor: Peter 66 Pmewood 3,817,211 6/1974 Brown et a1. 118/630 Weybfldge England 3,827,400 8/1974 Grenfall 118/D1G. 5

[22] Filed: June 7, 1974 Primary ExaminerLou1s K. Rimrodt [21] ApPl- Nod 477,349 Assistant ExuminerDouglas Salser [30] Foreign Application Priority Data ABSTRACT Apr. 23, 1974 United Kingdom 17668/74 A fluidized bed forming apparatus is provided which includes a filter extending partly across a base to de- [52] US. Cl. 118/308; 118/DIG. 5; 118/309; fine a plenum chamber beneath the filter. At least two 118/612; 118/630 end base portions are not above the plenum chamber [51] Int. Cl. B05c 7/14 and no fluidization occurs above those portions. [58] Field of Search 118/308, 309, 421, 612, Means are provided for supplying powder on to the 118/630, DIG. 5 base to form a fluidized bed which is retained by the unfluidized powder over those end base portions and [56] References Cited else where either by solid walls or further walls of un- UNITED STATES PATENTS fluldlzed P 3,254,625 6/1966 Armstrong 118/612 11 Claims, 4 Drawing Figures F LUIDISED BED APPARATUS BACKGROUND OF THE INVENTION This invention relates to a fluidised bed apparatus. One, though not the only, form of the invention is a coating apparatus.

It is known to provide small articles of low thermal mass with protective or decorative coating of thermosetting plastics or thermo-plastics material by a method which includes heating the articles and dipping them into a fluidised bed of powdered coated material.

It is also known to coat such articles by alternately re-heating and dipping them in powdered material in a fluidised bed until a sufficient thickness of coating has been built up. Electrical components such as capacitors, thick and thin film circuit units, resistors, coils and rectifiers are typical of articles which can be coated in this way with a protective coating of an epoxy resin. Such articles do, of course, include terminal leads as well as the actual electrical component and it is of great importance that the coating terminates clearly at the junction between the terminal leads and the component body. It is permissible for the coating to extend a very short distance along the terminal leads, such short distance of coating being commonly called the trouserleg or pantleg.

Two circumstances exist in which electrical components are particularly difficult to coat. The first is when the components have leads extending from both ends. These components cannot be coated by known fluidised bed coating methods as such coating methods would coat one of the leads also. The second circumstance arises when the components are mounted by their leads (which extend from one end of each component only) in a carrier strip, for example a strip of paper. Such strips can be much longer than the fluidised bed in which the components are to be coated, thus making it impossible to lower the whole strip with the components mounted thereon into the bed.

One of the objects of the present invention is to provide a fluidised bed apparatus by means of which components mounted in a strip, or otherwise arranged for transportation in a continuous linear fashion, can be coated, and which can be adapted to enable components having leads extending from both ends to be coated. i

BRIEF SUMMARY OF THE INVENTION According to the present invention there is provided an apparatus for forming a fluidised bed, which comprises a base arranged to receive powder; a filter extending across part of the base but not across at least two base portions at opposite ends of the base, thereby to define a plenum chamber, a gas inlet leading into the plenum chamber; and means for supplying powder on to the base, whereby a fluidised bed is formed over the filter and unfluidised powder accumulates above the said at least two base portions thereby to retain the fluidised powder against movement out of the fluidised bed.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic side elevation ofa simplified embodiment of the invention which is in the form of a coating apparatus;

LII

FIG. 2 is a plan view of an embodiment of the invention illustrating a method of conveying components to be coated;

FIG. 3 is a section taken along line AA in FIG. 2; and

FIG. 4 is a diagrammatic view of part of an embodiment designed to coat components which have leads at both ends.

DESCRIPTION OF A PREFERRED EMBODIMENT FIG. 1 shows an apparatus in which is afluidised bed of particles 1 which are brought to and maintained in a fluidised state by air which passes through ari'inlet pipe 2 into a plenum chamber 3 and thence through a filter 4. The apparatus has a base 5, and it will be noted that the base extends some way beyond the ends of the filter 4. There is thus a dead area at each end of the base where no fluidisation can occur, and hence a volume of unfluidised powder at each of the bed. These volumes of unfluidised powder retain the fluidised powder within the area over the filter 4 and prevent any substantial quantity from flowing away to theends of base.

One side of the bed is defined by a wall rising above the height of the bed. (This wall is on the side of the bed which is behind the bed as viewed in FIG. 1, and is not shown in that Figure). The other side of the bed is defined by a wall 6 having a knife edge 7, at the top thereof. Powder is allowed to overflow from the bed over the knife edge, which thereby defines the height of the bed, and is collected in a second fluidised bed which is U-shaped as viewed in plan. The U has a central portion 8, and two side arms 9 and 10 which collect any powder spilling over from the ends of the bed. The U-shaped bed need only have a single outlet which leads to a powder reservoir.

Instead of the form of U-shaped bed illustrated one could use a bed on the same level as the first fluidised bed and separated therefrom by a U-shaped unfluidised region. In this case one would have a single base covered by a single filter, a U-shaped portion of the underside of the filter being blocked off to define the unfluidised' region mentioned above.

Powder lost from the bed 1 is compensated for by adding powder through an inlet 11. Insteadof having side walls, the powder can instead be retained at the sides by unfluidised powder, as in the case of the ends. If both side walls are dispensed with it is necessary to control the level of the bed by some other means than the knife edge 6, for example via a level sensing probe.

Components 12 to be coated are fed through the bed 1 in a longitudinal direction. The components are attached by their leads 13 to a strip 14 which may be of paper or other suitable material. The movement of the components through the bed tends to cause a trailing edge effect and to compensate for this base 5 is reciprocated longitudinally with thespeed in the direction oftravel of the components being greater. than the speed in the reverse direction. The base 5 is also vibrated transversely as by cams 39 in order to break up the air streams emerging through the filter 4 and thus assist in ensuring uniform fluidisation of the bed 1. The vibration and the reciprocation can be effected by rotating eccentric cams as 40 bearing on the base or an extension piece 42 therefrom. The operation of such cams is believed to be self evident and they are therefore not illustrated.

It is clearly important to ensure that the components are immersed in the bed 1 to a uniform depth. One way of ensuring this is to feed them in by means of the arrangement shown in FIGS. 2 and 3. FIG. 2 also shows the means for heating the components so that resin powder melts when the components enter the bed and adhere thereto. The arrangement includes a pair of continuous chains running around wheels 16, the components being fed between adjacent runs of the chains on the strip 14. The strip 14 is unwound from a supply roll 17 and wound up on a take-up roll 18. Radiant heaters 19 and fluidised beds 20 are alternately positioned along the length of the adjacent chain runs so that the components are alternately heated and coated with resin, thereby to form a multilayer coating.

The construction of the chains is shown in more detail in FIG. 3. Only the left hand side of the section is shown, the right hand being a mirror image of the left. Each chain comprises links 21 which are pivotally interconnected with one another in a conventional manner by means of pivots 22. Extending over the top and bottom of each link are the two arms of a U-shaped member 23. Secured to the base of the U is a pad 24 of silicon rubber or other suitable resilient material. The strip 14 in which the components 12 are mounted is held between the pads 24 and corresponding pads carried by the opposite chain run. The two chains, and hence the opposing pads are urged towards one another by spring loaded members 25 each of which is loaded by a tension spring 26 attached at one end to a stationary part of the apparatus. Each chain is supported by a rail 27.

FIG. 4 shows part of an apparatus for coating a component having leads extending from both ends. A component 30 having leads 31 is mounted so as to be rotatable about the axis of the leads by driving means 32. In the illustration the driving means is shown as a chuck which engages one lead, the other lead being freely rotatable within a holder. Many other forms of driving means could be used instead. Most, but not quite all, of the part of the component below the axis of the leads is immersed in a fluidised bed of the type illustrated in FIG. 1, the bed being bounded laterally, but not at its ends, by knife-edged walls 34. Thus, as the component rotates all of its surface passes through the bed except for a radially inner portion on each side face of the component. Powder is directed on to these side faces via a chute 33 having inclined surfaces. This powder also falls on to the trouserlegs. Powder is directed into the appropriate regions of the chute 33, from a suitable inlet device (not shown), which may, for example, comprise a linear vibrator with a deflector for deflecting powder to the sides of the vibrator prior to its exit therefrom.

The component 30 is suitably heated before entry into the bed and on passage through the bed is completely coated without any coating being produced in the leads except for the trouserlegs.

Although as illustrated the chute 33 and fluidised bed are shown as simultaneously coating the component 30, these could apply powder successively instead, with either chute 33 or the fluidised bed applying powder I claim:

1. An apparatus for forming a fluidised bed, which comprises a base arranged to receive powder; a filter extending across part of the base but not across at least two base portions at opposite ends of the base, thereby to define a plenum chamber; a gas inlet leading into the plenum chamber; and means for supplying powder on to the base whereby a fluidised bed is formed over the filter and unfluidised powder accumulates above the said at least two base portions thereby to retain the fluidised powder against movement out.of the fluidised bed.

2. An apparatus as claimed in claim 1, wherein the fluidised bed is retained entirely by unfluidised powder surrounding the bed.

3. An apparatus as claimed in claim 1, wherein the sides of the bed are defined by a pair of walls one of which has a knife edge at the top thereof to determine the level of the fluidised bed.

4. An apparatus as claimed in claim 3, wherein means for forming a second fluidised bed are positioned to receive material overflowing the knife edge and material escaping from the regions of unfluidised powder, the second fluidised bed having an outlet through which material from the second bed can pass to a powder reservoir.

5. An apparatus as claimed in claim 1, wherein means are provided for feeding articles to be coated by the powder in the bed longitudinally through the first mentioned fluidised bed.

6. An apparatus as claimed in claim 5, wherein the articles are passed first through a heating zone and then through the first mentioned fluidised bed.

7. An apparatus as claimed in claim 5, wherein the articles are passed successively through a plurality of alternately arranged heating zones and fluidised beds.

8. An apparatus as claimed in claim 5, wherein means are provided for reciprocating the base in a direction parallel to the direction of travel of the articles, a greater velocity being given to the base in the same direction as that in which the articles travel than is given to it in the reverse direction.

9. An apparatus as claimed .in claim 5, wherein the means for feeding the articles comprises a pair of chains each of which carries resilent material thereon so disposed that the articles are clamped between the resilient material carried by the one chain, means being provided for urging the resilient material on the one chain against the resilient material on the other chain.

10. An apparatus as claimed in claim 1, wherein the articles to be coated are components each having a lead wire extending from opposite faces thereof, further comprising means for rotating the components about an axis defined by the leads at a level such that the leads are just above the level of the fluidised bed, and comprising means for feeding powder on to a radially inner region of each face of the component which is not coated by passage through the fluidised bed.

11. An apparatus as claimed in claim 1, wherein means are provided for vibrating the base transversely. l 

1. An apparatus for forming a fluidised bed, which comprises a base arranged to receive powder; a filter extending across part of the base but not across at least two base portions at opposite ends of the base, thereby to define a plenum chamber; a gas inlet leading into the plenum chamber; and means for supplying powder on to the base whereby a fluidised bed is formed over the filter and unfluidised powder accumulates above the said at least two base portions thereby to retain the fluidised powder against movement out of the fluidised bed.
 2. An apparatus as claimed in claim 1, wherein the fluidised bed is retained entirely by unfluidised powder surrounding the bed.
 3. An apparatus as claimed in claim 1, wherein the sides of the bed are defined by a pair of walls one of which has a knife edge at the top thereof to determine the level of the fluidised bed.
 4. An apparatus as claimed in claim 3, wherein means for forming a second fluidised bed are positioned to receive material overflowing the knife edge and material escaping from the regions of unfluidised powder, the second fluidised bed having an outlet through which material from the second bed can pass to a powder reservoir.
 5. An apparatus as claimed in claim 1, wherein means are providEd for feeding articles to be coated by the powder in the bed longitudinally through the first mentioned fluidised bed.
 6. An apparatus as claimed in claim 5, wherein the articles are passed first through a heating zone and then through the first mentioned fluidised bed.
 7. An apparatus as claimed in claim 5, wherein the articles are passed successively through a plurality of alternately arranged heating zones and fluidised beds.
 8. An apparatus as claimed in claim 5, wherein means are provided for reciprocating the base in a direction parallel to the direction of travel of the articles, a greater velocity being given to the base in the same direction as that in which the articles travel than is given to it in the reverse direction.
 9. An apparatus as claimed in claim 5, wherein the means for feeding the articles comprises a pair of chains each of which carries resilent material thereon so disposed that the articles are clamped between the resilient material carried by the one chain, means being provided for urging the resilient material on the one chain against the resilient material on the other chain.
 10. An apparatus as claimed in claim 1, wherein the articles to be coated are components each having a lead wire extending from opposite faces thereof, further comprising means for rotating the components about an axis defined by the leads at a level such that the leads are just above the level of the fluidised bed, and comprising means for feeding powder on to a radially inner region of each face of the component which is not coated by passage through the fluidised bed.
 11. An apparatus as claimed in claim 1, wherein means are provided for vibrating the base transversely. 